There are generally two types of equalization that exist in the return path of a communication signal in accordance with the data over cable system (DOCSIS) specification: 1) Pre-Equalization and 2) Post-Equalization. Post-equalization generally refers to a process that happens only in a cable modem termination system (CMTS). During post-equalization, the CMTS begins with an initial equalizer (for example, all 0's except for the main equalizer tap which begins with a 1). The CMTS then adapts/modifies the equalizer coefficients (via a feedback process) to compensate for any linear distortion that the signal has encountered. Linear distortion of the signal might result from the network element (e.g. modem) transmitter, cable plant, or CMTS receiver. The CMTS resets the equalizer to its initial values prior to the start of each new burst regardless of which network element the burst is coming from. As a result, a substantial number of incoming symbols at the beginning of each burst are often received before the post-equalizer is adequately converged, e.g., 100 symbols may be required for adequate convergence for most cable plants. The number of symbols required for convergence depends upon several factors, such as the “whiteness” of the symbols that the equalizer is trained on and the degree of linear distortion impacting the signal.
The DOCSIS protocol deals with the large number of symbols required for convergence by allowing the operator to configure the preamble length and preamble sequence such that the post-equalizer can obtain convergence during a preamble. A longer preamble length allows the CMTS post-equalizer to better converge prior to actually receiving the data payload portion of the upstream burst. Since, the CMTS resets its post-equalizer to its initial value prior to each data burst, the operator must send longer preambles for ALL data types, e.g., Initial Maintenance (IM), Station Maintenance (SM), Short Data (SD), Long Data (LD), Advanced Short (AS), Advanced Long (AL), and Unsolicited Grant Service (UGS). The longer preambles introduce higher overhead associated with the data packets and can have dramatic impacts on very short data packets, such as Voice over Internet protocol (VOIP). Moreover, for post-equalization every burst must be lengthened with a longer preamble to allow the equalizer to converge prior to receiving the data burst. If the preamble is not lengthened the equalizer may still be training (attempting to converge) during the beginning portion of the data payload, which greatly increases the likelihood of errors during the data transmission.
Pre-equalization relies on the modems or network elements to transmit a corrected communication signal to compensate for distortions. Pre-equalization requires the CMTS to store the equalizer coefficients for a given network element and then send them to the network element. The network element may then pre-distort its transmitted signal in such a way as to compensate for the distortion received by the CMTS. As a result, the CMTS should not see any (or at least minimal) distortion with its post-equalizer when a network element is transmitting with its pre-equalizer values. The CMTS obtains the pre-equalization values by training its equalizer using the network element station maintenance bursts during periodic ranging requests, and then sends the correct equalizer values to the network element in a ranging response message back to the network element. Successful periodic ranging is required to happen at a rate of at least every 30 seconds and generally occurs between 10-20 seconds. Thus, for pre-equalization, the ranging Interval Usage Code (IUC) (3,4) preambles must be long while the data IUC (1, 5, 6, 9, 10, 11) can utilize short preambles, which results in lower overhead.
However with pre-equalization, when distortion changes, such as when connected devices such as televisions, cable set top boxes or cable modems are powered on or off, the system is unable to compensate for this change until the next station maintenance interval which could be as much as 30 seconds later. This means as much as 30 seconds worth of data transmission may be lost, which is a catastrophic event for a VoIP call. Conversely, for post-equalization, both ranging IUC (3, 4) and data IUC (1, 5, 6, 9, 10, 11) preambles are all long to allow the CMTS to retrain for every burst. This better assures that packets will not be lost when distortion changes within the plant but at the expense of greater overhead due to longer preambles for all the data bursts.